Final answer:
An agonist is a muscle that contracts to act while the antagonist is the opposing muscle that relaxes or elongates in response to the action. The option (A) is correct.
Step-by-step explanation:
An action, such as flexing the knee, is caused by the contraction of a muscle acting as an agonist and is opposed by a muscle acting as an antagonist. The correct answer to the options provided is A. Agonist, antagonist. Muscles work in pairs where the agonist muscle contracts to act, such as the hamstrings contracting to flex the knee. The antagonist, in this case, the quadriceps femoris, would oppose the action by relaxing or elongating as the knee is flexed. When extending the knee, these roles reverse: the quadriceps femoris become the agonists (contracting to extend the knee), and the hamstrings serve as antagonists.
Additional terms related to muscle function include synergists, which assist agonists, and fixators, which stabilize a muscle's origin. An example of a muscle pair working in opposition would be the biceps and triceps muscles in the arm. The biceps are the agonists for elbow flexion while the triceps are the antagonists, and the roles reverse for elbow extension. Therefore, option (A) is correct.
This question is not complete, Here I am attaching the complete question:
An action, such as flexing the knee, is caused by the contraction of a muscle acting as an ______ and is opposed by a muscle acting as an ______.
A. Agonist, antagonist
B. Antagonist, agonist
C. Synergist, antagonist
D. Antagonist, synergist